Process of recovering zinc.



Patented oct. 8,1918;

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am@ I@ atto/MIM? UNTTED STATES PATENT CEEICE. y

FREDERICK LAIsT, 0E ANACoNDA, M'CNTANA, AssmNoR -To ANACCNDA COPPERMINING ComPANT, or ANACONDA, MONTANA, A CORPORATION oE MONTANA.

i To all wh'omc't may concern.'

i' Be it knownthat I,l FREDERICK LAIsT,`a citizen of the United States,residing at Anaconda, in the county'of Deerlodgev andvState of Montana,have invented certain vnevs7 and useful Improvements ingProcesses'lurgical treatment of. silicious z1nc ores the presence of gelatinoussilica at certain stages 0f theprocess interferes seriously lwith theoperation, not only by cloggingthe filters,

but also by persistentlyretaining considerable'volumes of'richzinc-bearing solution, and thereby reducing the recovery of zinc. Thepresent invention is a process `whereby these difiiculties are largelyovercome and `eight to nine per cent.. of sulfuric acid, this i fis,

materially increased'yields-of zinc are obtained. A preferred embodiment'of the process is as follows:-- p ,4

' Concentrates containing for example about 33 per cent. zine, eight perentllead, '20

.per cent. iron and s1x .per Acentginsolublev matter, the lastcomprising about 4.5 per' Cent.` silica and 1,5 per cent. alumina,- areroasted in multiple' hearth furnaces in order to oxidize the sulfids tooxi'ds and sulfates. The roasted product, or calcine, is thenl leachedwith Ya'l solution containing about solution in a repetition ofthe'process being theA depleted electrolyte from the ,cells in which.the zinc is electrolytically precipitated.- Usually.4 approximately fivetons of leaching solution are used per ton ofcal-I cine.l The resultingsolution is then neutralized or nearlyneutralized with calcine, .andthereafter a small roportion of limestone is usually added.

treatment. i Practically, it.is necessary to effect completeprecipitation 0f the -s'ilica at this point, since it vWill otherwiseseparate during later operating stalgesand occasionA `a great dealoftrouble.

. he precipitation of slhcamay be rendered even more'complete by addingto the pulp, after the limestone addition, a small amount of zinc dust:

this reagent, either in resence or absence of copper salts, is very eective-for precipitating any residual tracesof dissolved silica.

he effect of this limestoneaddition is toestablish a slight a'lkalinecondition in the liquor, and therebyto l precipitate .in'gelatinousfor-1n any s1lica which has been dissolved during the acidapplication alea :ranuary 29, 191s. seriaiNoi 214,311.

PRCESS 0F RECOVERINGZINC.

.I Specicationot Letter-s Patent. atented Oct, 8, 1918 In presence ofcopper salts the cement copper 1s of course the active reagent.

The above-described operations of neutralizing the liquid andkprecipitating the silica are conveniently performed in Pachuca tankswith thorough stirring. At this point the solution should carry at least80 per cent. of the zinc content of the'original cal..

cine. The Pachuca ltanks are now discharged into Dorr settling tanks orthickeners, and the thickened residue, together with the recipitatedmatters, passes to the filters. he filtrate, together with the overflowfrom the Dorr thickeners, is a zinc solu- .tion usually containinsmallamounts of copper. and cadmium. iese metals 'are precipitated ybyzinc-in dust or other suitable form, the solution clarified by settlingor filtration and passed to the electrolytic cells. The enteringelectrolyte is a neutral solution containing about* eight per cent. ofzinc in the form of sulfate: thedepleted electrolyte flowing from theelectrolytic cells contains some two yper cent. of zinc as sulfate,together Vith about eight to nine per cent. of sulfuric acid, and isreturned to the leaching tanks asmentioned above. Lead anodes may belused and the current con lltlons are preferably those which have novbecome standard in thisoperation.

Inthe practice of the above' process there is obtained a filter cakewhich comprises the insoluble matter from the ore and the precipitatedgelatinous silica. This is in the form of a wet cake, usually containing42-45 percent. of moisture, which is of .course in reality a rich zincsulfate solution.

It is impossible to wash the cake free from zinc with an aino'unt ofWater within practicable limits, and the losses at this point may attain15 per cent. of the zincv content. o'f the ore:

Efforts have been made to reduce this loss l by removing the leach-liquor from the residue before neutralization and While stillcontaining about 0.5 per cent. of acid. The

acid pulp-Was passed through thickeners and filters, and yielded a dry,sandy. cake', containing only about 20 er cent of moisture and readilywashed. owever, the subsequent treatment of the acid solution withcalcine, limestone and zinc dust yielded an unworkable precipitate ofgelatinousA silica, this precipitate being slimy, slow-settling,

and producing a cake containing up to 80 per cent. of moisture. A' Thenecessity of precipitating. the gelatinou's silica in presence of thesandy ore' residue was 'therefore estab` lished. 1 According to Athepresent invention the mechanical difficulties due to the precipita# tionof gelatinous silica, as Well as the excessive losses of zlnc occasionedthereby, are

avoided by proceeding substantially as fol? lows The silica isprecipitated in presence of the ore-pulp, preferably in the manner firstdescribed above, that is, by.successive additions of calcine, limestone,and zinc dust, if required, the latter either with or Without coppersalts. The subsequent operations of thickening and filtration are easilycarried out'in apparatus of standard type, but yield as above stated afilter cake of lhigh moisture-content and rich in zinc.

This filter cake, which may be termed the primary filter cake and which'need not be washed in the press` is discharged from the.

press aud thoroughly pulped byyigorous stirring with a relatively largevolume of Water, this volume being preferably about equal to theoriginal volume of 'leach liquor,

although a greater or less volume may be used. purpose is preferably notless than five times the dry weight o'f the-filter cake.4 ThisVoperation results in correspondingly diluting the zinc sulfate solutionwhich is mechani cally carried by the pulp. The',dilute pulp is nowpassed through settlers or thickeners and filters, yielding a secondaryfilter cake, which contains only a small proportion of the soluble zinccarried by the primary cake.l

This secondary cake may be rejected.

The clear solution or filtrate is relatively dilute, and is produced in"sov great a volume as to render its return to the main operating cycleimpracticable. It is therefore sepa- The amount of water used for thisture.

rately Worked for its Zinc centena-the preferred method comprisingpurification by treatment with Zinc in dust or other sultable form,clarification by settling or filtration or both, and electrolysis Withinsolublel anodes,

as lfor example lead. tIt has been found practicable even With thisdilute solution to -recover economically 75 per cent. or upward ofthezinc values by electrlytic precipitation. The spent electrolyte mayeither be from the' residue, re-pulping the residue.

with Water and separating the 'result-ing dllute zlnc-bearmg solutiontherefrom, and recoverlng zinc from the two zmc-bearlng solutions. l

vdiscarded orvvorked 'upby any appropriate I2. In a process ofrecovering zinc from silicious calcine, the steps which compriseleaching the calcine with4 an acid liquor, neutralizing the liquor andvtherebyprecipitating silica in presence of thev insoluble residue,separating the zinc-bearing solution from the residue, re-pulping theresidue with water and separating the resulting dilut'e zinc-bearingsolution therefrom,V and separately recovering zinc from the --tWozinc-bearing solutions by elect'rolyzing said4 solutions with .insolubleanodes.

v In't'estimony whereof, I aiiix my signa-

